MEASUREMENT OF SOUND "TRANSMISSION LOSS
BY SOUND INTENSITY
A P r e l i m i n a r y Report
by
R.W. Guy and A. De Mey
C e n t r e f o r Bu i l di n g S t u d i e s
Concordi a U n i v e r s i t y , Montreal
ABSTRACT
The sound i n t e n s i t y t e c h n i q u e i s being implemented t o measure sound
t r a n s m i s s i o n l o s s a t t h e Cent r e f o r B u i l d i n g S tu d i e s a c o u s t i c s t e s t
f a c i 1i t y .
The use of t h e i n t e n s i t y t e c h n i q u e f o r t h i s purpose i s being i n v e s t i ­
g a t e d i n t h r e e main a r e a s ; v a l i d a t i o n w i t h r e s p e c t t o s t a n d a r d t e c h n i q u e s ;
d e t e r m i n a t i o n of a p p r o p r i a t e measuri ng p r o c e d u r e ; e x p l o i t i n g t h e a n a l y t i c a l
c a p a b i l i t i e s of t h e t e c h n i q u e .
This paper p r e s e n t s some p r e l i m i n a r y
f i n d i n g s with r e s p e c t to t h e s e a r e a s .
SOMMAIRE
Dans l e l a b o r a t o i r e d ' a c o u s t i q u e du Cent r e des é t u d e s sur l e b â t i m e n t ,
l a t e c h n i q u e de mesure de l ' i n t e n s i t é a c o u s t i q u e e s t i n t r o d u i t e a f i n de
d é t e r m i n e r l a t r a n s m i s s i o n du son.
A c e t e f f e t , l ' u t i l i s a t i o n de c e t t e t e c h n i q u e e s t examinée dans l e s
t r o i s domaines s u i v a n t s :
v a l i d a t i o n p a r r a p p o r t aux normes, d é t e r m i n a t i o n
d ' u n e p r o c é d u r e de mesure a p p r o p r i é e e t ^ e x p l o i t a t i o n des c a p a c i t é s a n a l i tiq u e s de^cette^méthode.
Cet a r t i c l e p r é s e n t e q u e l q u es c o n c l u s i o n s p r é l i ­
m i n a i r e s à c e t ég ar d .
25
1.
INTRODUCTION
T r a d i t i o n a l l y t h e sound t r a n s m i s s i o n l o s s o f a p a n e l o r wal l h a s been
measured usi ng t h e s t a n d a r d , c l a s s i c approach as d e s c r i b e d by t h e ANSI/ASTM
E90-81.
However, t h e nuner ous c o n t r a d i c t i o n s bet ween r e p o r t e d r e s u l t s
based on t h i s method [ 1] s u g g e s t s t h e need f o r f u r t h e r i n v e s t i g a t i o n and
t h i s i s b e i n g a c h i e v e d t h r o u g h t h e a p p l i c a t i o n o f t h e Sound I n t e n s i t y
Technique at t h e Cent r e f o r Bu i l d i ng S t u d i e s a t Concordi a U n i v e r s i t y ,
Montreal.
This new method h a s s e v e r a l a d v a n t a g e s , f o r exampl e: i t g i v e s t h e
t r a n s m i s s i o n l o s s d i r e c t l y w i t h o u t havi ng t o make c o r r e c t i o n s f o r t h e panel
a r e a and t h e a b s o r p t i o n o f t h e r e c e p t i o n room; i t e l i m i n a t e s t h e e f f e c t o f
f l a n k i n g t r a n s m i s s i o n ; no r e s t r i c t i o n s ar e p l a c e d on t h e c h a r a c t e r i s t i c s of
t h e r e c e p t i o n room, t h a t i s i t n e i t h e r h a s t o be r e v e r b e r a n t o r a n e c h o i c ;
t h i s f a c t e l i m i n a t e s t h e need of an a c t u a l t r a n s m i s s i o n l o s s s u i t e ,
a l t h o u g h c u r r e n t l y t h e e x i s t a n c e o f a t l e a s t o ne r e v e r b e r a n t chamber i s
exploited.
As opposed t o p r e s s u r e , i n t e n s i t y i s a v e c t o r q u a n t i t y and t h e r e f o r e
provides d i r e c t i o n a l inform ation.
In o r d e r t o me a s u r e t h e t r a n s m i t t e d
i n t e n s i t y t hr o u gh a s u r f a c e , on l y t h e component p e r p e n d i c u l a r t o t h e
s u r f a c e i s needed.
However, t o d e s c r i b e t h e power f l o w d i s t r i b u t i o n ,
d i r e c t i o n or t r a n s m i s s i o n , t h r e e d i r e c t i o n a l powerflow may be d e t e r mi n e d .
The r e l a t i v e c o n t r i b u t i o n s t o t h e t o t a l sound t r a n s m i s s i o n o f d i f f e r e n t
s e c t i o n s of t h e t e s t panel can be d e t e r mi n e d .
T h i s paper p r e s e n t s t h e e v a l u a t i o n p r o c e d u r e employed t o implement t h e
Sound I n t e n s i t y Technique.
The t e c h n i q u e was appl i e d t o t h e me as ur e ment o f
Sound Tr a n s mi s s i o n l o s s t h r ou g h a panel wi t h and w i t h o u t a b s o r b e n t l i n e d
r e v e a l ; t h e r e s u l t s t h u s o b t a i n e d were t h e n compared w i t h t h o s e o b t a i n e d
us i n g t h e s t a n d a r d appr oach.
In a d d i t i o n t h e e f f e c t of t h e l i n i n g was
s t u d i e d and t h e d i s t r i b u t i o n o f t h e i n t e n s i t y r a d i a t e d t h r o u g h t h e p an el
d e t e r mi n e d .
2.
METHODS TO MEASURE THE SOUND TRANSMISSION LOSS
The t r a n s m i s i o n l o s s i s g i v e n by:
TL - 10 l oq1()
(!,/ît )
(1)
where î ^ i s t h e i n c i d e n t i n t e n s i t y and f ^ t h e t r a n s m i t t e d i n t e n s i t y .
2.1
S t a n d a r d Approach
The s t a n d a r d method of meas ur i ng t h e Sound Tr a n s mi s s i o n l o s s of a
panel o r wal l i n v o l v e s t h e u s e o f two v i b r a t i o n - i s o l a t e d r e v e r b e r a t i o n
chambers t h a t ar e s e p a r a t e d p a r t i a l l y or c o m p l e t e l y by t h e p a r t i t i o n t o be
studied.
The t r a n s m i s s i o n l o s s i s t h e n :
TL = Lp s - Lp r + 10 l o g 1Q
26
(S/A)
(2)
where Lps and Lpf are re s p e c tiv e ly th e average sound pressure le v e ls (dB)
in th e source and re c e iv in g rooms, S (m2) th e p a r t i t i o n ' s surface area and
A (m ) the absorption of the receiving room.
I t is assumed that the sound f i e l d s in both rooms are d iffu s e and that
there i s no fla n k in g transmssion.
2.2
Sound In t e n s it y Approach
The determination of the transmission loss of a panel or wall is now
done through the d i r e c t determination o f both the i n t e n s i t y in c id e n t on and
transmitted through the te s t p a r t i t i o n .
The incident i n te n s ity I- can be calculated from the measured spaceaveraged sound pressure Pfms in the source room assuming the sound f i e l d is
d iffu s e [ 3 ] .
where p i s the density of a ir and c the speed of sound in a ir .
The
accuracy o f t h i s equation has been v e r i f i e d by (rocker et al [3] by the
d ir e c t measurement of the i n t e n s i t y through the aperture formed afte r
removal o f the t e s t p a r t i t i o n .
From equation (3) the fo llo w in g re la tio n s h ip between the in c id e n t
i n t e n s i t y level L^- and the space averaged sound pressure level Lpm can be
derived [ 4 ] .
The transmitted i n t e n s i t y I I t
is measured on the receiving side of
the panel as the i n t e n s i t y v e c to r 's component perpendicular to the panel's
surface.
The sound transmission loss is then calculated from:
TL ‘ LPra - 6 - L I t
where L j t i s th e tra nsm itte d i n t e n s i t y l e v e l .
27
<dB>
<5>
3. TEST FACILITIES
3.1
Transmission Loss S uite
The transm is sion loss s u it e o f the Centre f o r B u ild in g Studies
(C .B .S )at Concordia U n i v e r s it y c o n s ists of 2 re c ta n g u la r rooms of d i f f e r i n g
dimensions.
The la r g e r room, the source raom ' f o r a l l the repo rted
experiments has a volume of approximately 94m .
The smaller room, the
r e c e iv in g room in t h i s case has a volume o f about 32 m . The t e s t aperture
between the rooms has an area of 7.5m and the f a c i l i t y is shown in Fig. 1.
Fig. 1 General Layout o f Transm issio n Loss Suite
a t the Centre fo r Building Studies, Concordia University.
The Schroder c u t - o f f frequency is 250 Hz f o r the la r g e r room and
400 Hj f o r the smaller one.
D iffu s in g elements c o n s is t in g o f one r o t a t i n g and two s t a t i o n a r y
d i f f u s e r s were located in the source room, and fo u r s ta t io n a r y d i f f u s e r s
were lo c a te d i n th e r e c e p t io n room.
The t e s t f a c i l i t y i s described in d e t a i l by Lang et al [ 6 ] .
3.2
Test Wall
In o r d e r t o accomodate t h e panel s i z e t e s t e d , a heavy f i l l e r w all was
constructed in the t e s t aperture between the two rooms. The composition o f
th e w a ll i s g iv en in Fig. 2.
Fig. 2 Cross Section of Filler Wall at Bese Plate
As can be seen the f i l l e r wall co n sis ts o f two w a l l s , mounted one in
each room on t h e i r r e s p e c t iv e room's a p e rtu re and separated from each o th e r
by i n s u l a t i o n m a t e r ia l .
The STC value o f the complete f i l l e r wall was 60.
The t e s t panel was
cm. (15.5") deep reveal
splayed at 45° towards
remaining w all depth.
displayed in Fig. 3.
mounted flu s h to the source room, leaving a 39.4
on th e r e c e i v i n g s id e .
Tine aperture was f u r t h e r
the recep tio n room to minimize the e fect of the
Hie method o f i n s t a l l a t i o n o f th e t e s t panel i s
29
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MONTREAL:
OTTAWA:
Main Office
Merivale Bldg.,
90 Leacock Road,
7 Siack Road, Unit 4,
Pointe Claire, Quebec H9R 1H1 Ottawa, Ontario K2G 0B7
Tel: (514) 695-8225
Tel: (613) 225-7648
Telex: 05821691 b + k pclr
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71 Bramales Road,
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Telex: 06-97501
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Richmond, BC V6X 2 A9
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Telex: 04-357517
Fig.3
The t e s t panel
0 . 6 4 cm ( i" ) t h i c k .
M ounting
of
T® st
Pan»l
on
Filler
Wall
used was a 1 . 1 4 m x 1 . 1 4 m.
(45" x 45")
glass
panel,
During t h e e x p e r i m e n t s r e p o r t e d h e r e , t h e r e v e a l on t h e r e c e i v i n g s i d e
was l e f t e i t h e r bare or l i n e d wi t h a 2 . 5 4 cm ( 1 " ) , 5 . 0 8 cm (2") and 1 0 . 16
cm (4") a b s o r b e n t m a t e r i a l :
Gonafl ex-F (by B l a c h f o r d ) .
For m a t e r i a l
p r o p e r t i e s , s e e Table 1.
TASIE 1.
Absorption C o e f f i c i e n t s of Conaflex F as Supplied by t h e
Ma nufacturer.
Absorption C o e f f i c i e n t (%)*
Freq.
(Hz )
125
250
500
1000
2000
4000
F-100 (1")
5
19
57
88
96
87
F-200 (2")
28
68
90
98
98
96
*Test method ASTM C 423-66
Te st sample s i z e 72 s q u a re f e e t
♦Approximate v a l u e s d e r i v e d from c h a r t
♦A b s o rp t i o n c o e f f i c i e n t f o r Conaflex F-400
(4") not a v a i l a b l e
32
4. TEST PROCEDURE
4.1
Standard TL Measurement
White noise was generated in the source room by two loudspeakers
placed in the corners of the room opposite the te s t aperture.
The mean sound pressure levels in the source room were measured using
a r o t a t i n g microphone boom (B & K 3923).
Hie microphone described a plane
c i r c u l a r path at 70° from the horizontal and the length of the arm was 1.6
m, t h i s c o n fig u ra tio n was chosen so t h a t th e microphone cleared the w alls
and s ta tio n a ry d iffu s e rs by at least 0.8 m (1/4 wave length at the 125
centre frequency is 0.68 m). The minimum distance from the microphone to
speakers was 1 m. and the period o f a complete re v o lu tio n o f the microphone
was 32 seconds.
In the receiving room, the mean sound pressure level measurment was
performed in the same manner as in the source room, however because o f i t s
smaller size, the length o f the arm was changed to .95 m and the tu rn ta b le
was t i l t e d at 60° from the h o riz o n ta l.
The reverberation time in the re c e iv in g room was calcula ted from the
averaged decay points (16 per second and 50 decay samples) with the tu r n ­
ta b le in the same p o s itio n as described above and with the microphone
ro ta tin g .
A lin e a r regression analysis was used in the range -5 dB below
th e upper decay po in ts down to 10 dB above background l e v e l .
All measurements were computer c o n tr o lle d and fed to a t h i r d octave
analyser.
In t h i s case, the Sound In te n s ity Analyser type 2134/3360 from
Bruel and Kjaer.
4.2
Sound In t e n s it y Method
The incident i n t e n s i t y was calculated from the mean sound pressure
le ve l as measured by the reverberant room method described e a r l i e r .
The tra nsm itte d sound i n t e n s i t y was measured d i r e c t l y using th e B & K
Sound In t e n s ity Microphone Probe type 3519, using the fac e -to -fac e micro­
phone c o n fig u ra tio n .
The
microphones w ith 12 mm spacer were chosen
which gives a useful frequency range, of 125
to 5 k
with an accuracy
o f +_ ldB assuming a monople source
The in t e n s i t y radiated through the panel was measured at 5.08 cm. (2")
behind the surface employing an a rray o f 81 evenly d i s t r ib u t e d p o in ts over
the surface; t h i s choice of measuring parameters w i l l be discussed la t e r .
I t became obvious during th e p r e lim in a r y t e s t phase t h a t in th e presence o f
the reveal, the transmitted sound in t e n s i t y used in the determination of
the transmission loss had t o be measured on th e re c e iv in g room side o f the
reveal where i t merges into the f i l l e r w a ll's surface. The same array of
p o in ts was used.
33
During t h e measurements the microphone probe was mounted on a
mechanical t r a v e r s e system t h a t enabled t h e microphones t o be f i x e d during
each measurement i n t e r v a l .
I t was then moved by hand from point to p o i n t ,
although l a t e r developments w i l l i n c l u d e t h e automation o f t h i s t r a v e r s e .
All d a t a was s t o r e d on d is k t hrough t h e use o f t h e Remote I n d i c a t i n g
Unit ZH 0250 (B & K).
In order to avoid r e v e r b e r a n t f i e l d e f f e c t s on the i n t e n s i t y method
measurement accuracy t h e t h r e e n o n - p a r a l l e l wa ll s of t h e r e c e i v i n g room
were covered with a t h i c k aborbent m a t e r i a l : Conaflex F-400.
Naturally
this
material
was
removed
for
the
c or re sp o n d i n g
sound
pressure
measurements.
4.2.1
Es ti ma ti on of t h e Phase Er r o r s in t h e I n t e n s i t y Measurements
Given the experimental c o n d i t i o n s d e sc r ib e d above, the r e a c t i v i t y of
t h e sound f i e l d was determined in both measurement planes f o r t h e purpose
of e s t i m a t i n g the e r r o r s r e s u l t i n g from the phase mismatch between the two
measuring c h an n el s . The r e a c t i v i t y i s t h e r a t i o between t h e sound p r e s s u r e
and the sound i n t e n s i t y .
The e r r o r Le r , defined as the d i f f e r e n c e between the measured
intensity
from [ 8 ] :
level
(dB)
and the t r u e
intensity
I
L
= -10 log
(1 er
10
p2
rc
where Pf e (Pa) i s the sound p r e s s u r e in
level
(dB) can be c a l c u l a t e d
P ^
-A . )
*
‘ me
(dB)
(6)
a c ompletely r e a c t i v e f i e l d ,
O
I r e (W/m ) t h e a p p a r e n t , r e s i d u a l i n t e n s i t y a s s o c i a t e d with i t , and P^(Pa)
and I
p
(W/m ) r e s p e c t i v e l y , the act ual measured sound p r e s s u r e and
intensity.
The measurement e r r o r s as given by (6) were r e l a t i v e l y high (up to 3
dB) a t t h e extreme lower end of t h e f reque nc y range. However, f o r f r e q u e n ­
c i e s equal to or higher than 250 Hz, they were found to be l e s s than 1 dB
at 5.08 cm (2") from t h e t e s t panel and l e s s than .5 dB a t t h e r e c e i v i n g
room s ide of t he r e v e a l ,
I f the r e c e i v i n g room had not been l i n e d , the r e a c t i v i t y of the sound
would have been h i g h e r , thus i n c r e a s i n g t h e measurement e r r o r s even
further.
5. PRELIMINARY TESTS
Although t h e use of t h e sound i n t e n s i t y t e c h n i q u e f o r t r a n s m i s s i o n
l o s s measurement has been e s t a b l i s h e d by o t h e r s , experimental d e t a i l s of
t h e procedure a re s t i l l vague and l e f t up t o t h e u s e r .
For example, t y p i ­
c a l l y what r e l a t i o n s h i p e x i s t s between measuring point d i s t a n c e and i n c l u ­
ded r a d i a t i o n s u r f a c e a r e a .
The s o l u t i o n s a v a i l a b l e are q u i t e v a r i a b l e
34
Cops [ 5 ] , f o r example uses a mesh s i z e of 19.5 cm x 20. 75 cm with a
measurement d i s t a n c e of 4 cm, wh i l e Fahy [4] us e s t h e same d i s t a n c e b u t f o r
a mesh of 4.5 cm x 7.5 cm. Th e r e f o r e, b efore the actual t r a n s m i s s i o n loss
t e s t s i t was n e ce s s a r y t o determine t h e s e paramet er s e x p e r i m e n t a l l y .
During t h e s e measurements only t he t r a n s m i t t e d i n t e n s t i t y l evel was
d e t e r mi n e d , t h e i n c i d e n t power b e i n g h e l d c o n s t a n t f o r each o f t h e s e r i e s .
5.1
I n f l u e n c e of Absorbent Material in t h e Reception Room
I n t e n s i t y measurements were made with and without absorbent ma t e r i a l
in t h e r e c e p t i o n room.
In t h e f or mer c a s e , t h r e e n o n - p a r a l l e l w a l l s o f t h e
r e c e p t i o n room were covered with Conaflex F-400.
As expected the val ues of the measured t r a n s m i t t e d i n t e n s i t y l e v e l s
f o r t h e u n l i n e d c a s e were lower t h a n t h o s e o b t a i n e d in t h e p r e s e n c e o f t h e
absor bent ma t e r i a l although t he d i f f e r e n c e s were very small with a maximum
d i s c r e p a n c y o f 1 db.
However, in o r d e r t o avoid any e f f e c t o f t h e r e v e r b e r a n t f i e l d on t h e
measurement accuracy involvi ng sound i n t e n s i t y measurements the r e c e p t i o n
room was always l i n e d as d e s c r i b e d .
5.2
Averaging Time
The averaging time i s an important parameter in
p r o c e d u r e both f o r acc ur a c y and f o r t o t a l d u r a t i o n o f t e s t .
t o minimize time without l os s of accuracy.
a measurement
The o b j e c t was
For an a rr a y of 81 p o i n t s t h e t r a n s m i t t e d i n t e n s i t y was measured using
d i f f e r e n t l i n e a r a ver aging t i m e s : 4 , 8, 16 and 32 s e c .
The r e s u l t s o b t a i n e d
were compared with the 32 sec. averaging time which was deemed a c c u r a t e for
s t e a d y s t a t e measurement.
The i n t e n s i t i e s averaged o v e r t h e t o t a l t e s t p a n e l s s u r f a c e were v e r y
s i m i l a r in a l l cases
.5 dB). However, a f t e r a comparison of t h e r e s u l t s
p o i n t f o r p o i n t , a l i n e a r aver ag i n g t i me of 8 s e c . was chosen s i n c e t h e
maximum poi nt d e v i a t i o n from t h e 32 sec. measurement did not exceed ldB.
5.3
Mesh Size
Four d i f f e r e n t mesh s i z e s were t e s t e d 38.1 cm x 38.1 cm (15" x 15"),
22. 86 x 22.86 cm (9" x 9 " ) , 16.33 x 16.33 cm (6.5" x 6 . 5 " ) , 12.7 x 12.7 cm
(5" x 5") giving a t o t a l nunber of measuring po i n t s of r e s p e c t i v e l y 3 x 3
( 9 ) , 5 x 5 (25), 7 x 7 (49), 9 x 9 (81) e v e n l y d i s t r i b u t e d over t h e t e s t
p an el 's surface.
No attempt to i n c r e as e t h e t o t a l nunber of po i n t s has
been made because o f t h e t ime p e n a l t y i n c u r r e d .
Each t i me t h e power flow
was measured at the c e n t r e of t h e subarea so c r e at e d at a d i s t a n c e , ( t e s t
panel t o c e n t r e o f microphone p a i r ) o f h a l f t h e mesh s i z e .
The r e s u l t s wi t h r e s p e c t t o t h e aver age t r a n s m i t t d i n t e n s i t y show t h e
fo l l o w i n g (Fig. 4):
r... i
___I___I___L
r
1
3B 1cm
x
5B
Icm
MESH
D=:9
□ 5 crr
2
22
x
22
9cm
MESH
D=I 1
Me m
5
IS
Ecm
x
16
Ber n
ME5H
D=9
5cm
M
12
7cm
x
12
7cm
ME5H
D=E
35cm
___L
50 0
FREQUENCY
Fig
4
9cm
1000
(Hz)
T h e I n f l u e n c e o f t h e M e s h S i z e on t h e M e a s u r e d
Transmitted Intensity
(tgv = 8 s e c )
L i t t l e difference is seen between the results of the 7 x 7 and 9 x 9
meshes, also with one exception differences were less than .5 dB.
For the 5 x 5 mesh, the only large deviation observed was around the
coincidence frequence in the 2500 Hy th ir d octave band. The peak in the
transmited in te n s ity , which gives rise to the coincidence dip in a trans­
mission loss p l o t , i s seen to be much lower and wider.
The r e s u lts o f the 3 x 3 mesh were more ir r e g u la r together with large
differences from the smaller mesh sizes.
For the present purpose, the smallest mesh size was chosen to avoid
inaccuracies and because of the more detailed information possible with
respect to establishing the contours of radiated in te n s ity d is tr ib u t io n .
5.4
Measurement Distance
In order to optimize the measurement distance the transmitted
in t e n s ity was measured at several distances from the t e s t panel.
With regard to the 9 x 9 mesh the distances chosen were
( 1 .5 " ) , 5.08 cm (2" ) s 7.62 cm (3"), 10.16 cm (4") and 12.7 (5").
Certain trends in the results can be observed (Fig. 5).
36
3.81 cm
125
250
500
FREQUENCY
Fia. 5
10 00
2000
HOOD
(Hz)
I nfluence of M e a s u r e m e n t Dist anc e
12.7
cm x
12.7
cm Me s h
t av
= 8
sec
When t h e d i s t a n c e i s s mal ler t ha n 10 cm ( 4 " ), t h e r e i s v e r y l i t t l e
d i f f e r e n c e (+ .6 dB) between r e s u l t s .
However, U
i n c r e a s e s with i n c r e a ­
sing d i s t a n c e below c o i nc i denc e . This t re nd r e v e r s e s above c o i nc i d enc e .
The l a r g e r t he measurement d i s t a n c e t he l e s s prominent t he c oi ncidence
peak, with t h e measured c o i n c i d e n c e f r eq u e n c y f i n a l l y f a l l i n g t o t h e n e xt
lower t h i r d octave frequency band.
As a consequence t he measurement d i s t a n c e chosen was 5.08 cm ( 2 ") .
6. TRANSMISSION LOSS TESTS
6. 1
Comparison of Standard and I n t e n s i t y - B a s e d Tr ansmissi on Loss
Measurement
For t h e sake of comparison between t he two measurement methods and in
o r d e r t o t a k e account o f the_ r e v e a l e f f e c t , t h e t r a n s m i t t e d i n t e n s i t y
r e p o r t e d in t h i s s e c t i o n was measured on the r e c e p t i o n room side of the
reveal.
37
125
350
500
FREQUENCY
Fig. 6
1
PRES5URE
2
I NTENSI TY
1000
2000
4 0 GC
(Hz)
C o m p ari so n be tween S t a n dar d an d I n t e n s i t y - B a s e d
T r a n s m i s s i o n Loss M e a s u r e m e n t
12.7 cm x 12 . 7 c m Mesh
t
« 8 sec
As shown i n Fig. 6 f o r th e re ve a l l e f t bare t h e r e s u l t s obtained are
g e n e r a ll y ve ry s i m i l a r w ith a maximum d if f e r e n c e o f 2 dB.
Greater
d i f f e r e n c e s can be seen a t lower fre q u en c ies and t h i s i s p rob a b ly due t o
the small re ce p tio n room s iz e .
The same trends were observed w ith the
re vea l l i n e d .
( V e r a l l , one may conclude t h a t th e e a r l i e r re po rted technique
v a l i d i t y , Crocker et al [ 3 ] , Fahy [ 4 ] , and Cops [ 5 ] , has been demonstrated.
6.2
L in in g o f the Reveal
The tr a n s m it t e d
s id e o f the r e v e a l .
i n t e n s i t y was again measured on th e r e c e p t io n
room
Fig.
7 shows t h a t th e e f f e c t
of
lin in g
th ic k n e s s
increases
g r a d u a lly o v e r most o f the frequency range, peaking in e f f e c t between, 1KH
and 2KHZ.
125
250
500
1000
FREQUENCY
F l 9-
1
Influence
o f L. i n i n p
Transmission
2000
4QQG
<Hz>
the
Reveal
on t h e
Total
Loss
1 2 . 7 c m x 1 2 . 7 c m Mesh
t
av
=8
sec
feneral l y th e t h i c k e r th e l i n i n g , t h e h ig h e r t h e measured tra n s m is s io n
l o s s , but only as a f u n c t i o n o f l i n i n g m a te ria l absorption c o e f f i c i e n t as
one might expect (see Table 1).
L i t t l e e f f e c t i s observed at th e lower frequencies and t h i s i s
p robably due to the low frequency absorption c h a r a c t e r i s t i c s o f the l i n i n g
m a t e r i a l , although t h e prominence o f g r a z in g mode tra n s m is s io n w i t h r e s p e c t
to the l i n i n g at low frequencies might also in flu e n c e t h i s r e s u l t .
When the tr a n s m itte d i n t e n s i t y i s measured d i r e c t l y behind the t e s t
panel a t a 5 cm (2") d is t a n c e , t h e measured tr a n s m is s io n l o s s i n th e cases
when the reveal was lin e d with 0, 2.54 cm (1") or 5.08 cm (2M) were the
same, as can be seen in Fig. 8.
Thererefore, th e tra n s m is s io n lo s s o f th e t e s t panel alone i s not
in flu e n ce d by the l i n i n g , and t h i s suggests th a t the panel v i b r a t i o n is
o n l y l o o s e l y coupled t o t h e a ir b o r n modes o f energy t r a n s f e r .
39
H
i
25
i
j ___i___L
i
25D
5 DG
1 DOD
FREQUENCY
F i 9-
8
Influence
Loss
of
Measured
12.7cm x 12.7cm
8.3
4GOO
(Hz)
Lininq
at
2 DOO
the
Me s h
the
Reveal
on
the
Transmission
Panel
D= 5 . 0 8 c m
(2")
t
= 8 sec
D i s t r i b u t i o n of t h e I n t e n s i t y Radi at ed Through t h e Te s t Panel
This t o p i c has been s t u d i e d t h e o r e t i c a l l y by f o r example Maidanek [7]
and r e c e n t l y also e x p e r i me n t a l l y by Fahy [4] using t he sound i n t e n s i t y
t echnique»
For f r e q u e n c i e s below c o i n c i d e n c e t h e t h e o r e t i c a l model
demonst r at es t h a t t he wave p a t t e r n at t he edges of a f i n i t e p l a t e cause
most r a d i a t i o n , in c o n t r a s t with an i n f i n i t e p l a t e .
It i s f u r t h e r
s uggested t h a t f o r t h e s e f r e q u e n c i e s only a s t r i p of p l a t e around the edges
r a d i a t e sound power,
/bove t h e c o i n c i d e n c e f r e q u e n c y , p a n e l s r a d i a t e from
t h e i r whole s u r f a c e , although t he experimental r e s u l t s of Fahy did not
c o mp l e t e l y a g r e e wi t h t h i s .
The b a s i c experiment ha s been r e p e a t e d ; t h e r a d i a t e d i n t e n s i t y was f o r
t h i s purpose measured d i r e c t l y behind the t e s t panel at a d i s t a n c e of 5.08
cm (2") and c o n t o u r s o f equal normal i n t e n s i t y were t h e n p l o t t e d ; r e s u l t s
a r e shown at 250
(Fig. 9), and 5000
(Fig. 10).
At v e r y low f r e q u e n c i e s ( Fi g. 9) t h e panel i s r a d i a t i n g pred o mi n a n t l y
through the c o r n e r s .
The i n t e n s i t y t r a n s m i t t e d through a small c e n t e r
p o r t i o n o f t h e panel i s much lower and t h e r e f o r e n e g l i g a b l e .
40
V
F ig . 9
Intensity Contours Norme! to t h e Panel
S u rfa ce at 2 5 0 Hz
At mid fre q u en c ies c lo s e r t o th e coincidence frequency i t was found
t h a t the center p o rtio n c o n t r ib u t i o n becomes l a r g e r , however greater
i n t e n s i t y i s found around th e panel border and th e g ra d ie n ts are found t o
be much steeper at the edges than at lower frequency.
Closer to and at the
co incidence fre q u en cy th e s trong i n t e n s i t y around t h e border o f t h e p la t e
remains, but the center p o rtio n o f the panel tends to ra d ia te much more
than a t lower fre q u e n c ie s .
Fig. 10 Intensity Contours Wormal to the ^®ns! Surfac© at §@@© Mg
41
Above the coincidence frequency (Figure 10) a q u it e uniform r a d i a t i o n
over th e whole s u r fa c e o f th e panel can be observed.
6.4
Fa u lt Finding
The c a p a b i l i t i e s o f the sound i n t e n s i t y technique with regard to the
d e t e c t i o n o f c o n s t r u c t i o n o r m a t e r i a l d e f i c i e n c i e s was a ls o examined.
F i® . 11
Ss to om o o f F a u lt
S trip Le n g th
0 .5 cm ;
I n t r o d u c e d by R e m o v i n g
C ra ck W id th
0 .2 m m
W o a th a rs tr ip p in g .
(a p p ro x im a te
d im e n s io n s )
For t h i s purpose a f a u l t was introduced by removal o f th e weatherst r i p p i n g on both sides o f the panel as shown in Fig. 11.
The exposed
p o r t i o n revealed a crack approxim ately 9.5 cm long and 0.2 mm wide between
the panel edge and i t s mounting frame.
I n t e n s i t y measurements were made d i r e c t l y behind the t e s t panel at a
d is t a n c e o f 5.08 cm ( 2 " ) .
The i n t e n s i t y p a t t e r n o b ta in e d was i n v e s t i g a t e d
f o r observable i r r e g u l a r i t i e s .
I t was found t h a t close to the f a u l t , the
values o f tr a n s m i t t e d i n t e n s i t i e s were g e n e r a l l y h ig h e r than a t th e same
p o in ts before the f a u l t was introduced.
The d if f e r e n c e s in local i n t e n s i t y
were s l i g h t a t lower f r e q u e n c i e s , up t o 3dB a t 250 Hz , i n c r e a s in g t o 10 dB
at 2000
and f a l l i n g lower again beyond t h i s fre q u en cy.
As can be seen in Fig. 12 the e f f e c t i s in d ic a te d by the i n t e n s i t y
co n to urs.
However, when comparing th e o v e r a l l sound tra n sm is sio n lo ss
before and a f t e r the i n t r o d u c t i o n o f the f a u l t (see Figure 13) the
i n f l u e n c e o f t h e f a u l t i s o n l y n o t i c e a b l e above 800 H7 and leads t o a
maximum d i f f e r e n c e i n o v e r a l l tra n s m is s io n lo s s o f 2.5 dB at 1KHZ w i t h
sm a lle r, to n e g l i g a b le d i f f e r e n c e s over th e r e s t o f th e fr equency range.
Such a f a u l t could
spectrum alone.
ea sily
be overlooked
42
by c o n s id e ra tio n
of the o v e r a ll
<dB>
LOSS
TRANSMI SSI ON
FREQUENCY
Fig. 13
(H z)
Comparison between the Transmission Loss of th e Test Panel
before and a f t e r the Introduction o f the Fault
12.7cm x 12.7cm Mesh
t a v = 8 sec
43
D = ^*08cm ^ " )
CONCLUSION
The v a l i d a t i o n o f t h e i n t e n s i t y - b a s e d t r a n s m i s s i o n l o s s measurement
has been c o n f i r m e d .
A d e t a i l e d measurement p r o c e d u r e has been e s t a b l i s h e d ,
and t h e a n a l y t i c a l c a p a b i l i t i e s o f t h e new method e x p l o i t e d t o d e t e r m i n e
t h e i n f l u e n c e of l i n i n g t h e r e v e a l of t h e t e s t panel with a b s o r b e n t
m aterial.
The o v e r a l l t r a n s m i s s i o n l o s s has been shown t o i n c r e a s e with
increasing
thickness
of
absorbent
lining.
However t h e
intensity
measurement t e c h n i q u e i n d i c a t e s t h a t t h e panel r a d i a t i o n i s no t i n f l u e n c e d
by t h e p r e s e n c e of t h e l i n i n g , such a c o n c l u s i o n would not have been
p o s s i b l e employing t h e s t a n d a r d r e v e r b e r a t i o n room t e c h n i q u e .
I t was a l s o d e m o n s t r a t e d t h a t t h e i n t e n s i t y t e c h n i q u e can be used t o
i d e n t i f y t h e e x i s t a n c e of untoward sound t r a n s m i s s i o n p a t h s as p a r t of a
normal measurement p r o c e d u r e .
ACKNOWLEDGEMENT
This work was s u p p o r t e d from i n d i v i d u a l o p e r a t i n g
Na ti o na l S c ie n c e and E n g i n e e r i n g Research C o u n c i l .
grant
A0374
of
the
REFERENCES
[1]
R.W. Guy, A. De Mey, P. S au e r , "The E f f e c t of Some P h y si c al P ar am et er s
Upon t h e L a b o r a t o r y Measurement of Sound T ra n s m i s s i o n Los s", C. B.S.
Repo rt No. 1 0 5 . , October 1983.
[2]
ANSI/ASTM
E90-75,
"Laboratory
Measurement
T r a n s m i s s i o n Loss o f B u i l d i n g P a r t i o n s " .
[ 3]
M.J. C r oc ke r, P.K. Raju, B. F o r s s e n , "Measurement o f T r a n s m i s s i o n Loss
of
P an el s
by t h e D i r e c t
D e t e r m i n a t i o n of T r a n s m i t t e d A c o u s t i c
I n t e n s i t y " , Noise Control E n g i n e e r i n g , Vol. 17, J u l y - August 1981,
p. 6-11.
[4]
F.J.
Fahy,
"Sound
I n t e n s i t y Measurement of
P r o c e e d i n g s o f t h e I n s t i t u t e o f A c o u s t i c s , 1982,
[5]
A. Cops, " Ac o u s t i c I n t e n s i t y Measurements and t h e i r A p p l i c a t i o n t o t h e
Sound T r a n s m i s s i o n Loss of P an el s and W a l l s " , I n t e r n o i s e 83, P.
567-570.
[6]
M. A.
Lang,
J.M.
Rennie,
"Qualification
of
a 94
Cubic
Meter
R e v e r b e r a t i o n Room Under ANS S I . 21 " , Noise Contr ol E n g i n e e r i n g /
September - October 1981, P. 6 4- 70 .
[7]
R.H. Lyon, G. Maidanik, "Response o f Ribbed P anel s t o
A c o u s t i c F i e l d " , J . A . S . A . , 1962, Vol. 34, P. 623.
[8]
Per Rasmussen, "Phase E r r o r s in I n t e n s i t y M e as u re me t s" , B & K A p p l i c a ­
t i o n Note, May 1984.
uu
of
Airborne
Sound
T r a n s m i s s i o n L os s ",
P. B5.1 - B5.4.
a Reverberant